The present invention relates to a proximity switch of the high-frequency oscillation type, which generates a detection signal when an object to be detected approaches the switch.
A conventional proximity switch of the high-frequency oscillation type has a detection coil in a detection head and the detection coil acts as the resonance coil of an oscillation circuit. An object is detected in terms of the drop in the output from the oscillation circuit.
FIG. 1 shows an oscillation circuit of the current feedback type for such a proximity switch. As shown in FIG. 1, a capacitor 2 is connected in parallel with a detection coil 1 provided in the front portion of the proximity switch, so that a resonance circuit 3 is constituted. An electrical current is applied to the resonance circuit 3 by a power supply 5 through a constant current circuit 4. One terminal of the power supply 5 is connected to a transistor 6 so that current amplification is performed. The emitter of the transistor 6 is grounded through a variable resistor 7 which functions to determine the magnitude of the collector current of the transistor. A current mirror circuit 12 including transistors 10 and 11 connected to emitter resistors 8 and 9, is connected to the collector of the transistor 6. The magnitude of an electrical current obtained from the resonance circuit 3 is subjected to the current amplification by the transistor 6. The magnitude of an electrical current, which is the same as that of the collector current of the transistor 6, is subjected to current feedback to the resonance circuit 3 through a transistor 11 by the current mirror circuit 12. Since positive current feedback is thus performed, oscillation is started at the resonance frequency of the resonance circuit 3. A smoothing capacitor 13 is provided to control power supplied to the resonance circuit 3. The oscillation stoppage speed of the oscillation circuit can be changed by altering the form of the detection coil 1 the number of turns thereof (for example, changing) or the other constants of the circuit. When an object to be detected approaches the proximity switch, the conductance of the detection coil 1 increases so that the the oscillation circuit oscillating stop oscillating. When the object goes away from the proximity switch, the oscillation begins again. The sum of the speed of the oscillation stoppage and the oscillation growth of the oscillation circuit is the response speed of the proximity switch.
Although the speed of the oscillation stoppage of the high-frequency oscillation circuit described above is generally high, the circuit has a drawback in that the speed of the oscillation growth of the circuit is very low. Thus, the high-frequency oscillation circuit disadvantageous that when the detection coil is increased in size, the response speed of the circuit decreases rendering impossible to accurately detect the approach of a quickly moving or changing object.
In order to solve the above-mentioned problem, an invention was made and disclosed in the Japanese Patent Application (OPI) No. 35619/86 (the term "OPI" as used herein means an "unexamined published application"). According to the invention, oscillation does not stop but continues at a low level when an object to be detected approaches a detection coil. However, the low level at which the oscillation continues despite the approach of the object must be exact and cannot fluctuate due to the change in the ambient temperature. Therefore, the invention is a disadvantages in that it is not practical for a proximity switch used in a rough environment.